The present invention relates to a "passive interrogator label system" (PILS); that is a system comprising an interrogator for transmitting an interrogation signal, one or more "labels" or passive transponders which produce a reply signal containing coded information in response to the interrogation signal, and a receiver and decoding system for receiving the reply signal and decoding the information contained therein.
A passive interrogator label system of the type to which the present invention relates is disclosed in the U.S. Pat. No. 3,273,146 to Horwitz, Jr.; U.S. Pat. No. 3,706,094 to Cole and Vaughan; U.S. Pat. No. 3,755,803 to Cole and Vaughan; and U.S. Pat. No. 4,058,217 to Vaughan and Cole. In its simplest form, the systems disclosed in these patents include a radio frequency transmitter capable of transmitting RF pulses of electromagnetic energy. These pulses are received at the antenna of a passive transponder and applied to a piezoelectric "launch" transducer adapted to convert the electrical energy received from the antenna into acoustic wave energy in the piezoelectric material. Upon receipt of a pulse, an acoustic wave is generated within the piezoelectric material and transmitted along a defined acoustic path. Further "tap" transducers arranged at prescribed, space intervals along this path convert the acoustic wave back into electric energy for reconversion into electrical energy by the launch transducer. The presence or absence of tap transducers at the prescribed locations along the acoustic wave path determines whether a reply pulse will be transmitted with a particular time delay, in response to an interrogation pulse. This determines the informational code contained in the transponder reply.
When an acoustic wave pulse is reconverted into an electrical signal it is supplied to an antenna on the transponder and transmitted as RF electromagnetic energy. This energy is received at a receiver and decoder, preferably at the same location as the interrogating transmitter, and the information contained in this response to an interrogation is decoded.
In systems of this general type, the informatin code associated with and which identifies the passive transponder is built into the transponder at the time that the tap transducers are deposited onto the substrate of piezoelectric material. As mentioned above, the presence or absence of tap transducers at prescribed locations along the acoustic wave paths determines whether a reply pulse will be transmitted with a particular time delay in response to an interrogation pulse. With this type of encoding, the number of possible codes is 2.sup.N where N is the number of tap transducers. For a large number of codes, it is necessary to provide a large number of tap transducers; however, increasing the number of tap transducers reduces the efficiency of energy conversion and introduces spurious signals into the reply signal.